1. Field of the Invention
This invention relates generally to wideband code division multiple access (WCDMA) for a communication system and more particularly to a system and method of data communication using transmit antenna diversity based on uplink measurements for the time division duplex (TDD) mode of WCDMA.
2. Description of the Prior Art
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver, i.e., mobile terminal, to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in U.S. patent application Ser. No. 9/205,029, filed Dec. 3, 1998, and incorporated herein by reference. Therein, Dabak et al. describe a method of space time transmit diversity (STTD) for frequency division duplex (FDD) WCDMA systems. These FDD systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames include pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (Tc), therefore, is equal to the symbol rate (T) divided by the number of chips in the symbol (G).
Time division duplex (TDD) provides an alternative communication standard for WCDMA, FDD systems. TDD data are transmitted as QPSK symbols in data packets of a predetermined duration or time slot. Each data packet includes a training sequence or midamble within the time slot. Data packets are exchanged within a cell formed by a base station in communication with nearby mobile units. Data in adjacent cells are modulated by different periodic codes. The midamble is formed by adding time shifted versions of the same basic sequence with each time shift corresponding to a mobile unit within the cell. The spreading factor (SF) or chips per symbol of the modulation is preferably sixteen or less. Since the periodic code within the cell is the same and the spreading factor is small, however, interference from the base station and other mobile units within the cell is not received as Gaussian noise. Typical matched filter circuits used in FDD systems, therefore, are unsuitable for eliminating this intra cell interference. A solution to this problem was presented by Anja Klein et al., Zero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code-Division Multiple-Access Channels, IEEE Transactions on Vehicular Technology, 276-287 (1996). Therein, Klein et al. teach zero forcing (ZF) and minimum mean-square-error (MMSE) equalization with and without decision feedback (DF) to reduce both inter-symbol interference (ISI) and multiple-access interference (MAI). Klein et al. further cites P. Jung, J. Blanz and P. W. Baier, Coherent Receiver Antenna Diversity for CDMA Mobile Radio Systems Using Joint Detection, Proc. IEEE Int. Symp. Pers. Indoor and Mobile Radio Communications, 488-492 (1993), for the proposition that these techniques may be used in combination with antenna diversity.
Delay diversity is a method of transmit antenna diversity in which the same signal is transmitted from multiple antennas, with each antenna having a different time delay. Delay diversity was introduced for TDMA systems such as IS-54 and GSM (global system for mobile communications) by N. Seshadri and J. Winters, Two Signaling Schemes for Improving the Error Performance of Frequency-Division-Duplex (FDD) Transmission Systems Using Transmitter Diversity, Vehicular Technology Conference, pp. 508-511 (1993) and A. Wittneben, A New Bandwidth Efficient Transmit Antenna Modulation Diversity Scheme for Linear Digital Modulation, International Communications Conference, vol. 3, pp. 1630-1634 (1993), wherein the delays for each antenna are chosen to be a multiple of the symbol interval. Delay diversity for a synchronous CDMA system (IS-95) was disclosed in U.S. Pat. No. 5,781,541, entitled CDMA System Having Time-Distributed Transmission Paths for Multipath Reception, by A. Schneider, issued Jul. 14, 1998 wherein the delays for each antenna are greater than a chip interval and less than the base station sequence offset between base stations. The Seshadri et al., Wittneben and Schneider references are incorporated by reference herein. Delay diversity for another CDMA communication system and using a distributed antenna system to provide multipath signals in order to facilitate signal diversity for enhanced system performance was disclosed by Gilhousen, et al. in U.S. Pat. No. 5,280,472, entitled CDMA Microcellular Telephone System and Distributed Antenna System Therefor, issued Jan. 18, 1994. A communication system having the advantages of delay diversity associated with TDD base terminals in combination with joint detection at the mobile terminals is, however, presently unknown. In view of the above, a wireless communication system having variable transmit antenna delays based on uplink measurements associated with the TDD mode of WCDMA in combination with joint detection of the transmitted signals at the mobile terminal is both advantageous and desirable.